Back in January, we reported that Apple acknowledges that certain Performa models (especially the 64xx and 65xx series) may have sound-related problems when on the Web. Specifically, shortly after the sound starts, the computers freeze up. So far, the issue seems to only occur with web sites that have embedded MIDI and WAV files. For some reason, the problem seems to have gotten worse in Mac OS 8.x.
Some users have linked this problem to the Geoport Express modem installed in some 6400 and 6500 models. A new web page describes more details. The page is seeking support to get Apple to replace these modems. As far as I know, Apple has not officially admitted that the modem is at fault. (Thanks, Sid Gabriel.)